Balloon catheters of various forms are commonly employed in a number of surgical procedures. These devices comprise a thin catheter tube that can be guided through a body conduit of a patient such as a blood vessel and a distensible balloon located at the distal end of the catheter tube. Actuation of the balloon is accomplished through use of a fluid filled syringe or similar device that can inflate the balloon by filling it with fluid (e.g., water or saline solution) to a desired degree of expansion and then deflate the balloon by withdrawing the fluid back into the syringe.
In use, a physician will guide the balloon catheter into a desired position and then expand the balloon to accomplish the desired result (e.g., clear a blockage, or install or actuate some other device). Once the procedure is accomplished, the balloon is then deflated and withdrawn from the blood vessel.
There are two main forms of balloon catheter devices. Angioplasty catheters employ a balloon made of relatively strong but generally inelastic material (e.g., polyester) folded into a compact, small diameter cross section. These relatively stiff catheters are used to compact hard deposits in vessels. Due to the need for strength and stiffness, these devices are rated to high pressures, usually up to about 8 to 12 atmospheres depending on rated diameter. They tend to be self-limiting as to diameter in that they will normally distend up to the rated diameter and not distend appreciably beyond this diameter until rupture due to over-pressurization. While the inelastic material of the balloon is generally effective in compacting deposits, it tends to collapse unevenly upon deflation, leaving a flattened, wrinkled bag, substantially larger in cross section than the balloon was when it was originally installed. Because of their tendency to assume a flattened cross section upon inflation and subsequent deflation, their deflated maximum width tends to approximate a dimension corresponding to one-half of the rated diameter times pi. This enlarged, wrinkled bag may be difficult to remove, especially from small vessels. Further, because these balloons are made from inelastic materials, their time to complete deflation is inherently slower than elastic balloons.
By contrast, embolectomy catheters employ a soft, very elastic material (e.g., natural rubber latex) as the balloon. These catheters are employed to remove soft deposits, such as thrombus, where a soft and tacky material such as latex provides an effective extraction means. Latex and other highly elastic materials generally will expand continuously upon increased internal pressure until the material bursts. As a result, these catheters are generally rated by volume (e.g., 0.3 cc) in order to properly distend to a desired size. Although relatively weak, these catheters do have the advantage that they tend to readily return to their initial size and dimensions following inflation and subsequent deflation.
Some catheter balloons constructed of both elastomeric and non-elastomeric materials have been described previously. U.S. Pat. No. 4,706,670 describes a balloon dilatation catheter constructed of a shaft made of an elastomeric tube and reinforced with longitudinally inelastic filaments. This device incorporates a movable portion of the shaft to enable the offset of the reduction in length of the balloon portion as the balloon is inflated. The construction facilitates the inflation and deflation of the balloon.
While balloon catheters are widely employed, currently available devices experience a number of shortcomings. First, as has been noted, the strongest materials for balloon construction tend to be relatively inelastic. The flattening of catheter balloons made from inelastic materials that occurs upon inflation and subsequent deflation makes extraction and navigation of a deflated catheter somewhat difficult. Contrastly, highly elastic materials tend to have excellent recovery upon deflation, but are not particularly strong when inflated nor are they self-limiting to a maximum rated diameter regardless of increasing pressure. This severely limits the amount of pressure that can be applied with these devices. It is also somewhat difficult to control the inflated diameter of these devices.
Second, in instances where the catheter is used to deliver some other device into the conduit, it is particularly important that a smooth separation of the device and the catheter balloon occur without interfering with the placement of the device. Neither of the two catheter devices described above is ideal in these instances. A balloon that does not completely compact to its original size is prone to snag the device causing placement problems or even damage to the conduit or balloon. Similarly, the use of a balloon that is constructed of tacky material will likewise cause snagging problems and possible displacement of the device. Latex balloons are generally not used for device placement in that they are considered to have inadequate strength for such use. Accordingly, it is a primary purpose of the present invention to create a catheter balloon that is small and slippery for initial installation, strong for deployment, and returns to its compact size and dimensions for ease in removal and further navigation following deflation. It is also believed desirable to provide a catheter balloon that will remain close to its original compact pre-inflation size even after repeated cycles of inflation and deflation. Other primary purposes of the present invention are to strengthen elastic balloons, to provide them with distension limits and provide them with a lubricious outer surface. The term "deflation" herein is used to describe a condition subsequent to inflation. "Pre-inflation" is used to describe the condition prior to initial inflation.